System and method for determining an absolute position of a motor shaft in an electric steering system

Abstract

A system and a method for determining an absolute position of a motor shaft in an electric power steering system during an ignition off state are provided. The system includes a microprocessor that energizes first and second position sensors to generate first and second signals, respectively, at a first time, and third and fourth signals, respectively, at a second time. The microprocessor determines a first relative position value based on the first and second signals, and a second relative position value based on the third and fourth signals. The microprocessor determines an amount of relative rotation of the shaft during the ignition off state based on the first and second relative position values, and determines a current absolute position value based on a previously stored absolute position value and the amount of relative rotation of the rotatable shaft.

What is claimed is: 
     
       1. A system for determining an absolute rotational position of a rotatable shaft of a motor in an electric power steering system of a vehicle, comprising:
 a microprocessor configured to be periodically activated to wake up from a low power sleep mode during an ignition off state of the vehicle by a timer circuit according to an activation duty cycle defined by a desired quiescent current draw of the microprocessor and a desired maximum speed of the rotatable shaft, the microprocessor further configured to energize first and second position sensors at a first time when the microprocessor is activated; 
 the first and second position sensors configured to generate first and second signals, respectively, indicative of a relative rotational position of the rotatable shaft at the first time; 
 the microprocessor further configured to measure the first and second signals and to determine a first relative position value indicating the relative rotational position of the rotatable shaft at the first time, and then to be deactivated, the microprocessor further configured to energize the first and second position sensors at a second time when the microprocessor is activated, the second time being after the first time and after the microprocessor was deactivated; 
 the first and second position sensors further configured to generate third and fourth signals, respectively, indicative of a relative rotational position of the rotatable shaft at the second time; 
 the microprocessor further configured to measure the third and fourth signals and to determine a second relative position value indicating the relative rotational position of the rotatable shaft at the second time, the microprocessor further configured to determine an amount of relative rotation of the rotatable shaft during the ignition off state based on the first and second relative position values, and the microprocessor further configured to determine a current absolute position value indicating a current absolute rotational position of the rotatable shaft based on a previously stored absolute position value and the amount of relative rotation of the rotatable shaft. 
 
     
     
       2. The system of  claim 1 , wherein the microprocessor is further configured to determine a total number of turns of the rotatable shaft of the motor by dividing the current absolute rotational position of the rotatable shaft by 360 degrees. 
     
     
       3. The system of  claim 2 , wherein the microprocessor is further configured to determine a total number of vehicle handwheel turns based on the total number of turns of the rotatable shaft of the motor and a gear ratio associated with the electric power steering system. 
     
     
       4. The system of  claim 1 , wherein the first and second position sensors are first and second Hall effect sensors, respectively, that are configured to measure a magnetic field of a magnet that is coupled to the rotatable shaft. 
     
     
       5. The system of  claim 4 , wherein the first and second Hall effect sensors are disposed 90 degrees apart from one another about a central axis of the rotatable shaft. 
     
     
       6. The system of  claim 1 , wherein the microprocessor is configured to energize the first and second position sensors at the first time by generating a control signal that induces a switch coupled to the first and second position sensors to supply an operational voltage to the first and second position sensors. 
     
     
       7. The system of  claim 1 , wherein a time interval between activations of the microprocessor is utilized to reduce power consumption of the microprocessor. 
     
     
       8. A method for determining an absolute rotational position of a rotatable shaft of a motor in an electric power steering system of a vehicle, comprising:
 periodically activating a microprocessor to wake up from a low power sleep mode during an ignition off state of the vehicle utilizing a timer circuit according to an activation duty cycle defined by a desired quiescent current draw of the microprocessor and a desired maximum speed of the rotatable shaft; 
 energizing first and second position sensors at a first time when the microprocessor is activated, utilizing the microprocessor; 
 generating first and second signals, respectively, indicative of a relative rotational position of the rotatable shaft at the first time utilizing the first and second position sensors, respectively; 
 measuring the first and second signals and determining a first relative position value indicating the relative rotational position of the rotatable shaft at the first time utilizing the microprocessor; 
 deactivating the microprocessor after determining the first relative position value; 
 energizing the first and second position sensors at a second time when the microprocessor is activated, utilizing the microprocessor, the second time being after the first time and after the microprocessor was deactivated; 
 generating third and fourth signals, respectively, indicative of a relative rotational position of the rotatable shaft at the second time, utilizing the first and second position sensors, respectively; 
 measuring the third and fourth signals and determining a second relative position value indicating the relative rotational position of the rotatable shaft at the second time utilizing the microprocessor; 
 determining an amount of relative rotation of the rotatable shaft during the ignition off state based on the first and second relative position values utilizing the microprocessor; 
 determining a current absolute position value indicating a current absolute rotational position of the rotatable shaft based on a previously stored absolute position value and the amount of relative rotation of the rotatable shaft utilizing the microprocessor; and 
 storing the current absolute position value in the memory device utilizing the microprocessor. 
 
     
     
       9. The method of  claim 8 , further comprising determining a total number of turns of the rotatable shaft of the motor by dividing the current absolute rotational position of the rotatable shaft by 360 degrees. 
     
     
       10. The method of  claim 9 , further comprising determining a total number of vehicle handwheel turns based on the total number of turns of the rotatable shaft of the motor and a gear ratio associated with the electric power steering system, utilizing the microprocessor. 
     
     
       11. The method of  claim 8 , wherein the first and second position sensors are first and second Hall effect sensors, respectively, that are configured to measure a magnetic field of a magnet that is coupled to the rotatable shaft. 
     
     
       12. The method of  claim 11 , wherein the first and second Hall effect sensors are disposed 90 degrees apart from one another about a central axis of the rotatable shaft. 
     
     
       13. The method of  claim 8 , further comprising energizing the first and second position sensors at the first time by generating a control signal which induces a switch coupled to the first and second position sensors to supply an operational voltage to the first and second position sensors utilizing the microprocessor. 
     
     
       14. The method of  claim 8 , wherein a time interval between activations of the microprocessor is utilized to reduce power consumption of the microprocessor. 
     
     
       15. A system for determining an absolute rotational position of a rotatable shaft of a motor in an electric power steering system of a vehicle, comprising:
 a comparator configured to compare first and second back electromotive force voltages from first and second phases, respectively, of the motor to a reference voltage, and to output a control signal when either the first back electromotive force voltage is greater than the reference voltage or the second back electromotive force voltage is greater than the reference voltage; 
 the microprocessor configured to be activated to wake up from a low power sleep mode in response to the control signal; 
 the microprocessor further configured to be periodically activated to wake up from the low power sleep mode by a timer circuit according to an activation duty cycle defined by a desired quiescent current draw of the microprocessor and a desired maximum speed of the rotatable shaft; 
 the microprocessor further configured to energize first and second position sensors at a first time when the microprocessor is activated; 
 the first and second position sensors configured to generate first and second signals, respectively, indicative of a relative rotational position of the rotatable shaft; 
 the microprocessor further configured to measure the first and second signals and to determine a first relative position value indicating a relative rotational position of the rotatable shaft at the first time; 
 the microprocessor further configured to measure the first and second signals and to determine a second relative position value indicating a relative position of the rotatable shaft at the second time, the second time being after the first time; 
 the microprocessor further configured to determine an amount of relative rotation of the rotatable shaft during the ignition off state based on the first and second relative position values; and 
 the microprocessor further configured to determine a current absolute position value indicating a current absolute rotational position of the rotatable shaft based on a previously stored absolute position value and the amount of relative rotation of the rotatable shaft. 
 
     
     
       16. A method for determining an absolute rotational position of a rotatable shaft of a motor in an electric power steering system having a power on state and a power off state, comprising:
 periodically activating a microprocessor to wake up from a low power sleep mode during the power off state of the system based on one or more of: a timer circuit according to an activation duty cycle defined by a desired quiescent current draw of the microprocessor and a desired maximum speed of the rotatable shaft and a difference in back electromotive force voltage from first and second phases, respectively, of the motor to a reference voltage, wherein the activating of the microprocessor to wake up from the low power sleep mode is responsive to both the timer circuit and the difference in back electromotive force voltage; 
 determining a first relative position value indicating the relative rotational position of the rotatable shaft at a first time during the power off state utilizing the microprocessor; 
 deactivating the microprocessor after determining the first relative position value; 
 determining a second relative position value indicating the relative rotational position of the rotatable shaft at a second time during the power off state utilizing the microprocessor; 
 determining an amount of relative rotation of the rotatable shaft during the power off state based on the first and second relative position values; 
 determining a current absolute position value indicating a current absolute rotational position of the rotatable shaft by comparing an absolute position value stored during the power on state and the amount of relative rotation of the rotatable shaft utilizing the microprocessor; and 
 storing the current absolute position value in the memory device. 
 
     
     
       17. The system of  claim 15 , wherein the microprocessor is configured to be activated in response to the control signal from the comparator based on a rotational speed of the rotatable shaft being above a threshold rotational speed. 
     
     
       18. A system for determining an absolute rotational position of a rotatable shaft of a motor in an electric power steering system having a power on state and a power off state, comprising:
 a microprocessor configured to be periodically activated to wake up from a low power sleep mode during the power off state of the system by a timer circuit according to an activation duty cycle defined by a desired quiescent current draw of the microprocessor and a desired maximum speed of the rotatable shaft, the microprocessor further configured to energize a plurality of position sensors when the microprocessor is activated; 
 the plurality of position sensors configured to generate at least a first signal and a second signal indicative of a relative rotational position of the rotatable shaft at a first time; 
 the plurality of position sensors further configured to generate at least a third signal and a fourth signal indicative of a relative rotational position of the rotatable shaft at the second time, the second time being different than the first time; and 
 the microprocessor further configured to determine an amount of relative rotation of the rotatable shaft during the power off state based on a difference between the relative rotational position at the first time and the relative rotational position at the second time, and the microprocessor further configured to determine a current absolute rotational position of the rotatable shaft based on an absolute position value stored during the power on state and the amount of relative rotation of the rotatable shaft. 
 
     
     
       19. The method of  claim 16 , wherein activating of the microprocessor is responsive to the difference in back electromotive force voltage based on a rotational speed of the rotatable shaft being above a threshold rotational speed.